1. Field of the Invention
The present invention relates to an X-ray inspection device and an X-ray inspection method which are capable of detecting a foreign matter formed of a particular element in a sample.
2. Description of the Related Art
In recent years, a lithium-ion secondary battery, which has a higher energy density compared with that of a nickel metal hydride battery, has been increasingly adopted as a battery for an automobile, a hybrid car, an electric vehicle, and the like. The lithium-ion secondary battery is a kind of a non-aqueous electrolyte secondary battery, which has lithium ions in an electrolyte conducting electricity and does not contain metal lithium, and has already been used widely in notebook personal computers and mobile phones.
The lithium-ion secondary battery has excellent battery characteristics. However, when a foreign matter such as Fe (iron) enters an electrode during a production process, the reliability of the battery is adversely affected, such as degradation in battery characteristics as to a heat-generation property, a longevity, and the like, which has delayed the mounting of the battery on a vehicle. For example, as illustrated in FIG. 4A, an electrode (anode) of the lithium-ion secondary battery is generally formed in such a manner that a lithium cobalt oxide film 2 or a lithium manganese oxide film 2 is formed to a thickness of about 100 μm on both surfaces of an Al film 1 having a thickness of 20 μm. As illustrated in FIG. 4B, a foreign matter X such as Fe (iron) or SUS (stainless steel) may be mixed into the electrode, and if the size of the foreign matter X is tens of pm or larger, short-circuit occurs, which may cause burning of the battery and decrease in performance. Therefore, there is a demand in the lithium-ion secondary battery that a battery with the foreign matter X mixed therein be detected swiftly during production and be removed in advance.
In general, as a method of detecting a foreign matter or the like in a sample, a method using an X-ray image is known. Using this procedure, a method of detecting a foreign matter formed of a carboneous material, in which the presence/absence of the contamination of a foreign matter in a carbon-based material or the like used as a cathode of a lithium-ion secondary battery is detected based on an X-ray image, has been proposed conventionally (see Japanese Patent Application Laid-open No. 2004-239776 (claims)).
The above-mentioned related art has the following problem.
Specifically, with the conventional method of detecting a foreign matter, the presence/absence of a foreign matter is merely detected based on the intensity of an X-ray image. Therefore, if the atomic number of a foreign matter is largely different from that of a sample, a clear contrast is obtained. However, if the atomic number of the foreign matter is close to that of the sample, a contrast is not clear, which makes discrimination therebetween more difficult. For example, there is no clear contrast between Co (atomic number: 27) contained as one of constituent elements in an electrode (positive plate) to be a measurement sample and Fe (atomic number: 26) that is an element constituting a foreign matter to be detected. Therefore, with the conventional method of detecting a foreign matter, it is impossible to determine whether a contrast is caused by a portion 2a where a constituent material is partially thick as illustrated in FIG. 4A or caused by the foreign matter X as illustrated in FIG. 4B in an X-ray image T of an electrode (positive plate) that is a measurement sample S, which may lead to misdetection or erroneous detection.